Note: Descriptions are shown in the official language in which they were submitted.
~159972
This invention relates to a tw~-wire requency diYision
mult:iplex system.
A two-wire frequency division multiplex system enables the
transmission of multi-channel telephone messages, e.g. telephone messages,
in b~th directions on the same pair of wires by using different frequency
bands for the transmitting and the receiving directions. The frequency
bands are divided in smaller channel bands each of which contains
the modulated information of the predetermined one of the multiplexed
channels respectively.
Some existing two-wire frequency division multiplex systems
comprise two terminal stations A and B, connected by a two wire line,
which operate on the same frequency channels ~baseband) but each
of ~he stations contains additional frequency conversion equipment
connected to either the transmit or the receive line terminals. Thus,
station A may have its transmission converted to a different frequency
band (sub-group) to be sent over the line and converted back to the original
baseband for reception by station B. The process may be reversed by
reconnecting the converter sections at both the terminal stations. A
common sub-group carrier is used for both conversions.
Regarding narrow choice of sub-groups, it may be difficult
to achieve the up and down conversion with sufficient degree of lineari~y
such as to avoid generation of intermodulation products which would interfere
with operation of the system. The problem may be particularly acute
when close harmonics of one sub-group fall inside the frequency band
of the other.
Further, conventional frequency division multiplex systems
suffer rom higher distortion and interference in one direction than
in the other because a signal from station A to station B has to
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pass throu~h two conversion stages before reaching the recipient,
wh0reas a return signal from B to A will be received directly on the
baseband frequency. Thus conversion stages have to be built to exscting
specifications in order to guarantee the minimum quality standard
for 'both directions.
The present invention provides a simpler two-wire frequency
division multiplex system which comprises first and second kinds of
multiplex stations connected by two-wire lines, and distinguished
by means to both produce and to receive a predetermined one of two
different predetermined frequency basebands each of which consists
of predetermined frequencies for dividing the multiplex channels,
said first kind of multiplex stations including transmit means having
conversion means to covert their own basebands to the different basebands
of the second kind of multiplex stations and vice-versa.
In the said configuration signals transmitted in either
direction pass through only one frequency conversion equipment to
divide the transmitted and received frequencies, thereby reducing
noise and intermodulation and allowing for less expensive type of
converter. The conversion equipment can be operated at a constant
and optimum level, thereby allowing the mixers to be properly balanced
and further reducing the harmonic and intermodulation conten~ of the
transmitted spectrum. Further, the two multiplex stations may be
identical as regards layout and wiring, particularly when employing
multiplex equipment with a fixed pre-modulation frequency, where interchange
of the station functions may be accomplished simply by interchanging
the baseband carrier frequency generators and the filter terminals
of the hybrid line combiners.
The invention will now be further discussed in conjunction
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with the accompanying drawings, in which:
Figure 1 is a block diagram illustrating the conventional
two-~tire frequency division multiplex system with conventional frequency
conversion,
Figure 2 is a spectrum diagram of the frequencies used in
the conventional two-wire frequenc~ division multiplex system of Figure
1,
Figure 3 is a block diagram of a two-wire frequency division
multiplex system according to the invention,
Figure 4 is a frequency spectrum diagram of a two-wire frequency
division multiplex system according to the invention and as shown
in Figure 3,
Figure 5 is a simplified block diagram of station A of the
two-wire frequency division multiplex system according to the invention,
Pigure 6 is a simplified block diagram of station B with
different frequency basebands in comparison to Station A in the two-
wire frequency division multiplex system according to the invention,
Figure 7 is a spectrum diagram of the configuration of station
A according to the invention,
Figure 8 is a spectrum diagram of the configuration of station
B sccording to the invention,
Figure 9 is a spectrum diagram of the transmission spectrum
together with the pilot frequencies according to the invention.
Referring to Pigure 1 and Figure 2 there is shown a conventional
two-wire frequency division multiplex system which provides the same
baseband frequencies in terminal station A and in terminal station
B, and converts this baseband in the transmit path of terminal station
A and reconverts the converted base~and in the receive path of terminal
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station B. The frequency sp0ctrum diagram of Figure 2 shows the voice
chamlel being first converted by the baseband carriers in order to
get the baseband sub-group which contains the particular channel requency
band.s each of which is achieved by modulation of its predetermined
baseband carrier. After a second conversion using sub-group carrier
frequencies the complete baseband is converted to another frequency
band labelled in the spectrum diagram as sub-group direction AB.
After the third conversion of the baseband in the receive path of
the terminal station B the normal baseband of the terminal station
is obtained and labelled as baseband sub-group or sub-group direction
BA. This conventional two-wire system has the above mentioned disadvantages.
Figure 3 shows the improved two-wire frequency division
multiplex system in accordance with the invention. In each of the
terminal stations A and B only one conversion of the complete baseband
is performed by the frequency converters placed in the transmit path
of each station.
As shown in the frequency spectrum diagram of Figure 4 station
A and station B have two different basebands produced by the baseband
A carriers and the baseband B carriers which are different for both
stations. The same sub-group carrier frequency is provided in station
A and station B to convert their own bssebands to those of the other
terminal stations. For example, the station A baseband and receive
baseband is converted to the station A transmit baseband being the
equivalent of the station B baseband and receive baseband. Similarly,
the station B transmit baseband is equivalent to the station A baseband
and receive baseband.
In a particular embodiment of this invention illustrated
in Figure 5 and Figure 6, station A ;s made to operate with four channels
-- 4 --
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in the baseband o 3~-48 kHz with station B operating with four channels
in the baseband of 8-24 kHz.
Each ststion is made up of the multiplex equipment, converter,
hybrid network and pilot facilities. The multiplex equipment employed
for this purpose is of a dou~le conversion type, utilizing a pre-modulation
band of 48-52 kHz and a pre-modulation carrier of 48 kHz.
With reference to Figure 5 and Figure 6, the multiplex equipment
comprises: four identical chsnnel dems with transmit section (1)
and receive section (2) each, one pre-modulation carrier generator
of 48 kHz (3), four channel carrier generators t4) selected for frequencies
of 84, 88, 92 and 96 kHz for station A and 28, 32, 36 and 40 kHz for
station B and one sub-group combiner each for transmit ~5) and receive
(6) directions tuned to the station baseband frequencies (32-48 kHz
for station A and 8-24 kHz for station B).
The frequency converter (7) is in the transmit path (down-
converter in station A and up-converter in station B) which is injected
with a sub-group carrier of 56 kHz from the generator (8).
A separation filter hybrid network (9) selects the frequencies
of the respective basebands of the receive and transmit paths connected
to the two-wire line.
A variable gain amplifier (10) in the receive path is controlled
by the output of the pilot receiver (11) operating at 48 kHz in station
A and at 8 kHz in station B serving the purpose of compensation for
the variable line losses.
All carrier generators including the pilots are phase-locked
to a common, crystal controlled master oscillator ~12~ with sufficient
long term stabilit~ to make the operation of the two stations A and
B independent. Facilities for alignment of the master oscillators
1~59~72
are omitted for clarity.
~ igure 7 shows the frequency spectr~ diagram of the configuration
of station A in accordance with t~e invention. Referring to line
(a), the voice frequencies VF are first converted by the pre-mod carrier
of 48 kHz and then converted by the channel carriers or baseband carriers
of 84, 88, 92 and 96 kHz, line Cb), to achieve the predetermined baseband
of station A within the frequencies of 32-48 kHz. In order to be
transmitted the baseband of station A is converted, line ~c), by the
sub-group carrier frequency of 56 kHz to form the baseband of station
B containing the frequencies of 8-24 kHz.
Figure 8 shows the frequency spectrum diagram of the configuration
of station B according to the new multiplex system. The voice frequencies
VF are at first converted, line (a), by the pre-mod carrier frequency
of 48 kHz as in station A. To achieve the predetermined baseband of
station B in the frequencies of 8-24 kHz further conversion is
performed by the channel carrier frequencies of configuration station
B with the frequencies of 28-40 kHz, line ~b). Thus, the different
basebands of station A and station B are only composed by the different
channel carrier frequencies of both stations. Before transmitting
the baseband of station B is converted by the same sub-group carrier
frequency of 56 kHz to that of station A with the frequencies of 32-
48 kHz. A preferable choice of the frequencies of both basebands
of 8-24 kHz and 32-48 kHz allows the use of a single sub-group carrier
frequency of 56 kHz in both stations A and B.
Regarding Figure 9 there is sho~n the complete transmission
frequency spectrum from station A to station B and vice-versa. The
pilot frequencies are added at the extremé ends of the spectrum, one
just below the low fre4uency end of the lower baseband and one just
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above the high requency end of the higher baseband. The~ can be
mixed with and separated from the signals in a conventional manner.
Provision of two pilot frequencies so placed ensures better compensation
of line characteristics throughout the transmission spectrum.
The 2-wire network system employing the improved configuration
is not limited to t~o terminals A and B. Generally, the applications
may be extended to a chain of links:
A ---- 8 ---- A ---- B ---- etc.
or as a star:
A ---- B '~--- A etc.
A
or to a combination thereof, provided type A stations will link directly
with type B stations only.
Communication between similar type terminals can be established
by voice frequency cross-channel looping at the intermediate stations.
